The present application claims priority to Japanese Application No. P2000-154049 filed May 25, 2000 which application is incorporated herein by reference to the extent permitted by law
The present invention relates to a method of fabricating a thin film transistor. More particularly, the invention relates to a technology for keeping clean the condition of an interface between a semiconductor thin film and an insulating film formed on the surface of the semiconductor thin film.
In recent years, thin film transistors have been extensively developed as active devices in active matrix circuits integratedly formed on an insulating substrate. The thin film transistor, for example, has a semiconductor thin film of polycrystalline silicon as a device region. In order to isolate individual thin film transistors, a semiconductor thin film of polycrystalline silicon or the like must be patterned in island forms by photolithographic technology.
However, there has been the problem that when polycrystalline silicon is directly patterned, deposition of a large amount of impurities uncontrollable in photolithographic step occurs, and the impurities would diffuse into semiconductor thin film or an insulating film in contact therewith in latter steps or would segregate to the surface of the semiconductor thin film. In such a situation, it is difficult to control the characteristics of the thin film transistor.
Countermeasure against the above problem is disclosed, for example, in Japanese Patent Laid-open No. Hei 10-116989. According to the disclosure, a first gate insulator is first formed on the surface of a semiconductor thin film consisting of polycrystalline silicon. Subsequently, the surface of the semiconductor thin film of polycrystalline silicon is patterned through the first gate insulator without exposing the surface to a photoresist treatment. Thereafter, only the surface of the first gate insulator is partly removed by etching. Further, a second gate insulator is formed on the first gate insulator. According to the prior art, however, although the layer of polycrystalline silicon is not exposed to the atmosphere and is kept clean, a contaminated interface is generated between the first gate insulator and the second gate insulator. With electric charges trapped at the contaminated interface, there may arise the problem that the characteristics of the thin film transistor cannot be controlled.
It is an object of the present invention to provide a technology of fabrication of a thin film transistor by which a semiconductor thin film constituting a device region of the thin film transistor can be patterned in island forms while keeping a clean surface condition. In order to attain the above object, the following means have been adopted. Namely, the present invention resides in a method of fabricating a thin film transistor having a laminated structure including a semiconductor thin film, an insulating film formed in contact with the surface of the semiconductor thin film, and a gate electrode disposed on the face side or the back side of the semiconductor thin film, and formed on a substrate in a predetermined plan view shape, the method including: a first step of forming a semiconductor thin film having a clean surface over the substrate; a second step of forming a protective film so as to cover the clean surface of the semiconductor thin film; a third step of patterning the semiconductor thin film together with the protective film according to the plan view shape of the thin film transistor; a fourth step of removing the protective film from the upper side of the patterned semiconductor thin film to expose a clean surface of the semiconductor thin film; and a fifth step of forming an insulating film in contact with the exposed surface of the semiconductor thin film.
Preferably, the first step includes forming a semiconductor thin film consisting of silicon being amorphous or being polycrystalline with a comparatively small grain size, followed by irradiation with laser light to convert the amorphous or polycrystalline silicon to polycrystalline silicon having a comparatively large grain size. The first step may comprise forming a semiconductor thin film consisting of silicon being amorphous or being polycrystalline with a comparatively small grain size, followed by a heat treatment to obtain polycrystalline silicon having a comparatively large grain size through solid phase growth. Besides, the second step may include forming a protective film consisting of an insulating substance. In this case, the second step may comprise thermally oxidizing the surface of the semiconductor thin film consisting of silicon to form a protective film consisting of silicon oxide. The second step may comprise forming a protective film by building up silicon oxide through chemical vapor deposition. Alternatively, the second step may comprise forming the protective film by building up a semiconductor substance. Preferably, the second step comprises forming a protective film having a thickness of not less than 5 nm. The fourth step may comprise removing the protective film by wet etching in which a chemical having a dissolving function is made to act in a flowing manner without circulatorily using the chemical. Preferably, the method comprises a sixth step of forming a gate electrode on the insulating film formed in contact with the clean surface of the semiconductor thin film.
According to the present invention, a protective film being, for example, insulating is formed on a semiconductor thin film consisting of polycrystalline silicon or the like. The semiconductor thin film is patterned together with the protective film by photolithography, whereby the surface of the semiconductor thin film is protected from contamination in the photolithographic step. Therefore, it is possible to keep clean the interface between the semiconductor thin film and the gate insulator formed on the surface of the semiconductor thin film.
According to the present invention, since the interface between the semiconductor thin film consisting of polycrystalline silicon or the like and the gate insulator can be kept clean, diffusion of impurities in a channel is suppressed, whereby mobility of the thin film transistor is largely improved. In addition, the amount of impurities in the gate insulator in contact with the semiconductor thin film or in an interlayer dielectric is reduced, whereby threshold voltage of the thin film transistor can be stabilized.